Method and apparatus for the lifting of offshore installation jackets

ABSTRACT

An offshore lifting construction is able to control buoyancy for the lifting of jackets for offshore platforms. The lifting construction is designed such that it can be ballasted for raising and lowering and such that it can take several positions. The lifting construction is designed as an arm equipped with a joint for attachment to a vessel. The arm includes two substantially parallel longitudinal elements with intermediate supporting elements. One of the ends of the arm includes at least two tubular branches substantially perpendicular to the longitudinal direction of the substantially parallel longitudinal elements. At least one of the elements of the arm includes at least one ballastable tank. The arm is equipped with device for attachment to the jacket that is to be lifted. A method for rising jackets or jacket legs for offshore platforms is also described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns vessels for lifting heavy loads offshore, andparticularly vessels for removing offshore platforms with steel jackets,after the platform deck is removed for instance by means of anotherlifting vessel. A method for using such vessels is also enclosed.

2. Description of Related Art

As offshore oilfields throughout the world are being depleted, and whenfurther exploitation no longer is profitable, platforms will have to bedecommissioned or shut down. It is required from various authorities andinternational cooperation treaties (OSPAR Convention decision 3/98 andthe Guidelines of the Maritime Organization IMO) that all platforms thathave been taken out of service, and that can be removed, shall bebrought onshore for removal.

On the Ekofisk field alone, soon 15 platforms will be decommissioned andremoved. Most of these platforms include a steel jacket on the seabedwith a platform deck on top.

The steel jackets are distinguished by being of a truss work designwhere the weight of the platform is transferred through the legs of thejacket down to the seabed, and from there further down through poles.These designs or constructions are relatively weak in a transversaldirection and do not have sufficient strength to transfer its own weightwithout many supporting points, localized on the joints or nodes betweenthe legs and the transversal supports. Typical for these constructionsare 4 to 12 legs with a weight of up to 20,000 tons.

A conventional method for the installation of such jackets is that theseare laid horizontally on a barge carrier and thereafter either lifted orpushed into the sea, rotated vertically, and lowered down to the seabedby means of a crane vessel rotating the jacket vertically before beinglowered to the seabed. If the jacket is too heavy, the filling ofbuoyancy tanks is used as a supplementary aid. During removal, thissequence can be reversed. However, it is a problem that such jacketsfrequently are difficult to turn, and this means that they often have tobe cut into smaller parts for transport to the shore. The conventionallifting vessels have a limited lifting capacity, and the use of suchrequires a lot of under water work. This work can include installationof new lifting lugs, cutting off legs and crossbeams and possibly theuse of buoyancy tanks either alone or in combination with a liftingvessel. Transferring the jacket to a barge for transport to the shorealso requires calm weather conditions with low wave heights, somethingthat possibly lengthens these operations and thereby makes them morefinancially demanding.

Other vessels of various designs have been suggested to simplify theseoperations.

Norwegian Patent Application 1996 5439 shows a transporting device forthe installation and removal of platform underframes or jackets. Theapplication shows that the transporting device is an elongatedconstruction with floating elements that can be ballasted such that thetransporting device takes a substantially vertical position in the sea.The transporting device is then led towards the jacket that is to belifted, and is secured to a sliding saddle on the transporting device.The transporting device is then deballasted such that it is lifted alongwith the jacket to a substantially horizontal position.

U.S. Pat. No. 4,651,667 shows a pivoted barge for the towing andlaunching of offshore constructions. A joint pivots as the construction,that is to be launched, is displaced along with its center of gravityand results in a pivoting movement of the joint.

The building of new vessels is however fairly expensive and necessitatesmany marine systems in addition to fabrication of steel material. At thesame time there are a large number of vessels tied up waiting for work.These may already be equipped with systems for safety, ballasting,mooring, towing and propulsion in addition to accommodation, cantinasand large deck areas. These systems have also been tested, certified andused. By attaching a lifting structure to an existing vessel, theexpenses for a lifting or decommissioning vessel is considerablyreduced. It is expected that an amount of between 5,000 and 10,000 tonsof new steel can be saved. Such existing vessels can be fairlyinexpensive, for instance due to the present situation in the North Sea.The integration between the existing and the new constructions can bekept to a minimum to control the expenses. Making a lifting constructionsuited for connection to different vessels makes it possible to keep theday rates to a minimum. By integrating these additional services in thelifting vessel, the need for additional vessels during the preparationoffshore will be limited.

SUMMARY OF THE INVENTION

Accordingly the present invention concerns an offshore liftingconstruction with means for controllable buoyancy for the lifting ofjackets or jacket legs for offshore platforms, where the liftingconstruction is designed such that it can be ballasted to be raised orlowered, and such that it can take a substantially horizontal raisedposition, and a substantially vertical lowered position. The liftingconstruction is designed as an arm or frame equipped with means forpivotal attachment to a vessel. The arm comprises two ends and at leasttwo substantially parallel elongated members with intermediate supportmembers or beams. One of the ends of the arm includes at least twosubstantially parallel elements or branches substantially perpendicularto the longitudinal direction of the substantially parallel elongatedelements. These elements may be tubular. At least one of the elements ofthe arm includes at least one ballastable tank. The arm is equipped withmeans for securing the jacket that is to be lifted.

Furthermore, the invention describes a method for the raising of jacketsor jacket legs for offshore platforms with the apparatus describedabove. The method includes the steps described below. The vessel withthe arm is navigated to the jacket of the offshore installation that isto be removed. The arm is ballasted by filling the ballast tanks in thearm with sea water, such that the arm sinks and is pivoted around thejoint, with a sequence in relation to the various ballast tanks suchthat the lowering is performed without the joint or the vessels beingloaded beyond certain limits, to a lowered, substantially verticalposition. The jacket is placed between the two branches. The jacket issecured with means for securing the jacket that is to be lifted, to thearm. The ballast tanks are then deballasted such that the arm is pivotedalong with the jacket about the joint until the arm is substantiallyhorizontal and the jacket is raised.

The arm of the invention is designed with two elongated tubes,preferably of steel, with beams in between. At the end of the arm thereare two tubes or branches substantially perpendicular to thelongitudinal direction of the arm. The arm can be water tight to make itbuoyant, and is divided into several tanks that can be filled or emptiedwith ballast water. The arm is adapted to be secured to a vessel in ajoint with the ability to rotate in a vertical plane (as compared to athwart ship axis). This joint can alternatively be a spherical jointwith a rotation additionally about the two other axis's (longitudinaland vertical) and if it is adequate buoyancy in the arm, the joint canslide in a vertical direction.

The branches are preferably adapted to provide sufficient buoyancy witha predetermined point of application, they are placed with aninterrelated distance such that the jacket can be provided between thebranches and they can serve as a support for the jacket. Alternatively,the internal distance between the branches can be adjusted to be adaptedto the lifting of jackets of various dimensions.

The intermediate supporting elements or beams on the arm, can form or beincluded with supporting points or cradles that are adapted to liealongside the jacket legs. The beams, the supporting points or thecradles are preferably adjustable such that they can be adjusted againstthe strong points of jackets of various sizes. The jacket can be securedto the arm in various ways, such as clamping, tack welding, strappingand bolting, but preferably the legs resting towards the frame aresecured to the top point against a construction or arrangement at thetop of the arm. This construction can include winches with wires thatare secured to the jacket, and can be adapted to lift the jacketsomewhat. This lifting is particularly useful during the first part ofthe raising or the last part of a launching when the jacket is to belifted from, or attached to, a foundation on the seabed. This liftingcan alternatively be performed with hydraulic cylinders between the armand the jacket.

The vessel must be deballasted to compensate for the weight of thejacket that is transferred through the arm and the supporting joint. Atthe same time, it is important to empty the buoyancy tanks enabling thearm to rotate towards a horizontal position.

The jacket legs that are to be removed must be detached or cut off atthe seabed before they can be removed. In the event of an upper platformon the jacket, this must also be removed before the jacket can belifted.

By filling ballast in the tanks, the arm will rotate to a verticalposition. The vessel and the arm is then led in towards the jacket suchthat the arm is lying towards the jacket. The beams on the arm, thesupporting points or cradles that are to be lying alongside the legs(the jacket legs) are in advance adjusted such that they rest againstthe strong points of the jacket, and are adjusted in relation to these.

The load on the construction at the top of the arm is reduced as theweight of the jacket is transferred to the arm. At the end, the jacketwill lie on the arm in a substantially horizontal position. The liftingconstruction according to the invention is built with sufficientstrength and buoyancy to lift platform jackets.

Vessels that can be used in connection with a lifting construction orarm according to the invention, can include vessels for drilling,accommodation, barges, semi-submersible vessels, vessels for particularpurposes, supply vessels, etc. If a barge is used, this will be securedthrough the joint. The joint can then be secured in a skidding shoe thatcan skid along a skidding rail. The skidding rail can be designed suchthat the skidding shoe not can be lifted up and out of the rail duringoperation. The skidding shoe can also alternatively be temporarilylocked against horizontal motion. This can be performed by means of alocking plate/bolt arrangement. The joint can include a bolt goingthrough a bearing that is secured to, or forms a part of, the skiddingshoe and the arm.

The arm can then be designed to enable it to be drawn over the barge bymeans of winches. It can, for instance, be two winches installed at thefront part of the barge deck, one on the starboard and one on the portside. When the arm is to be drawn over the barge, the wires are securedto the front part of the arm, the locking arrangement for the skiddingis released, and the skidding operation can begin. Alternatively, thedevices can be used to lead the arm over the barge. These devices mayinclude hydraulic cylinders or rack and pinion systems.

The arm may then be filled with ballast such that it steadily liestowards the barge before the arm and the jacket is secured. Othermethods for securing the arm and the jacket must be evaluated dependingon weather conditions, distance to shore, etc.

If a different lifting vessel is used, where the arm cannot be pulledover the vessel, the jacket will float on the lifting arm that issecured to the vessel for transport to shore. In this case, the skiddingequipment is not necessary.

The ballast system is designed such that each tank can be filled andemptied separately. Ballasting is performed by means of submerged pumpsor pressurized air. Pumps and valves are controlled from the vessel, andalternatively from a supporting vessel. Ballasting should preferably beperformed such that a controlled load is applied to the joint and suchthat it is ensured that the arm is pivoting in the right direction ascompared to the vessel during rising. The branches could for instance,during rising, be ballasted first, such that the point of attack orpoint of application of the buoyancy creates a rotating force or momentabout the joint such that the arm is turned the correct way duringrising. When the arm has turned a certain angle as compared to thehorizontal in this way, ballast tanks in a lower tubular transversalsection can be emptied, succeeding emptying of the ballast tanks in thelongitudinal tubes.

The jacket can be shipped onshore with trailers. The vessel can bemoored with the bow towards a quay with the deck in the same height asthe quay. The trailer can be placed below the beams on the arm. Thebeams can then be led to or from the arm before the trailers lift thebeams with the jacket on or off.

The advantages with the invention is that it results in a lowfabrication cost, no activities under water, except for cutting off orin any other way releasing the jacket from the foundation, and securingthe jacket above the waterline. The invention can be adapted to allsteel jackets, be secured to all barge sizes, and does not normallyrequire being adapted to different barges, with possibly the exceptionof the pivoting joint, where the skidding rail must be installed. Thisresults in a quick, simple, safe and thereby cost effective removalmethod, with a simple unloading.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of one embodiment of an offshore liftingconstruction according to the invention, attached to a barge, where thelifting construction lies in a substantially horizontal position;

FIG. 2 shows a perspective view of the embodiment shown on FIG. 1, in asubstantially vertical position, secured to a steel jacket that is to beraised;

FIG. 3 shows a perspective view of the embodiment shown on FIG. 1, in asubstantially horizontal position, secured to a steel jacket that hasbeen raised;

FIG. 4 shows a perspective view of the embodiment shown on FIG. 1, in asubstantially horizontal position, secured to a steel jacket that israised, where the barge is led in under the lifting construction;

FIG. 5 shows a side elevation of another embodiment of an offshorelifting construction according to the invention, attached to a vesselwith a unshaped end section, where the lifting construction lies in asubstantially vertical position secured to a steel jacket that is to beraised or lowered;

FIG. 6 shows a front elevation of FIG. 5;

FIG. 7 shows a top elevation of FIG. 5;

FIG. 8 shows a side elevation of the embodiment shown FIG. 5, but wherethe lifting construction lies in a substantially horizontal position;

FIG. 9 shows a front elevation of FIG. 8;

FIG. 10 shows a top elevation of FIG. 8;

FIG. 11 shows a perspective view of a detail of a joint between a bargeand a lifting construction according to the invention;

FIG. 12 shows a top elevation of the details on FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 to FIG. 4 shows an embodiment of an offshore lifting constructionor arm (1) according to the invention. The figures show various phasesof a lifting process. The lifting construction is shown attached to abarge (2). The lifting construction is designed with two substantiallyparallel longitudinal elements or tubes (5), between, on the upper partof the lifting construction provided, a frame (4) with a holdingarrangement (9) for the jacket that is to be lifted, two intermediatesupporting elements or beams (6), a lower tubular transversal unit (7)and a joint (10) for attachment to the barge (2). On the lower part ofthe two substantially parallel longitudinal elements (5), two tubularelements or branches (8) are provided substantially perpendicular to thelongitudinal direction of the substantially parallel longitudinalelements (5). The barge is equipped with a skidding rail (11) andwinches (12). The joint (10) (shown on FIG. 1 and FIG. 11) is skiddinglyconnected to the skidding rail (11).

The elements of the lifting construction (1) form or comprise internalballast tanks (not shown) that can be ballasted by pumps for instance onthe barge, in the lifting construction, or on a supporting vessel. Theballast tanks are preferably placed several places in the arm (1) suchthat the size and the point of application of the buoyancy can becontrolled.

On FIG. 1 it is shown that the lifting construction (1) lies in asubstantially horizontal position and creates an elongation of the barge(2).

From FIG. 2, it is shown that the lifting construction (1) is ballastedand is thereby turned or pivoted about joint (10) (shown on FIG. 1 andFIG. 11) and is lowered to a substantially vertical position. Thelifting construction (1) and the barge (2) is maneuvered against ajacket (13) that is to be lifted. The jacket (13) that is to be liftedis shown secured to the holding arrangement (9). The jacket is placedbetween the branches (8) on the lower part of the two substantiallyparallel longitudinal elements (5). The two intermediate supportingelements (6) may be adjustable or include adjustable supporting points(not shown) that can be adjusted such that they can rest against strongpoints (14) on the jacket (13) that is to be lifted. The holdingarrangement (9) includes winches that can pull the jacket (13).

From FIG. 3, it is shown that the lifting construction (1) isdeballasted and is thereby pivoted about the joint (10). The legs (15)of the jacket (13) are cut off or in any other way released from itsfoundations at the seabed. The jacket (13) and the lifting construction(1) lies substantially parallel to the barge (2).

From FIG. 4, it is shown that the lifting construction (1) along withthe jacket (13) is led over the barge (2). The barge (2) is led underthe lifting construction (1) by pulling the barge (2) in below the arm(1) by means of the winches (12) with wires (16) secured to the liftingconstruction (1), such that the joint (10) has skidded along theskidding rail during the guidance of the barge (2) under the arm (1). Inthis embodiment the arm (1) is secured with a joint (18) to a vessel (3)with a u-shaped bow section. The vessel (3) can be a semi submergiblevessel. The u-shaped end may be an integrated part of a vessel, orconstitute a module attached to another vessel. The construction caninclude holding elements (not shown) to hold the arm (1) attached to thevessel (3) when this is raised or, in other positions, if required.

The next embodiment is shown with three intermediate supporting elements(6) that are adjustable or includes adjustable supporting points (notshown) that can be adjusted to lie against strong points (14) on thejacket that is to be lifted.

FIG. 5 shows a side elevation where the arm (1) is ballasted and isthereby pivoted about a joint (18) and is lowered to a substantiallyvertical position. The lifting construction (1) and the barge (2) aremaneuvered against the jacket (13) that is to be lifted. The jacket (13)is also here placed between the branches (8) on the lower part of thetwo substantially parallel longitudinal elements (5).

FIG. 6 is a front elevation of FIG. 5 where the three intermediatesupporting elements (6) are adjusted or in any other way placed suchthat they rest against strong points (14) on the jacket that is to belifted.

FIG. 7 shows a top elevation of FIG. 5 where the u-shaped bow section ofthe vessel is clearly shown.

FIG. 8 is a side elevation of the embodiment on FIG. 5, but where thelifting construction (1) is deballasted and is pivoted to asubstantially horizontal position.

FIG. 9 shows a front elevation of FIG. 8 where the jacket (13) is partlyplaced inside the u-shaped bow section.

FIG. 10 shows a top elevation of FIG. 8.

FIG. 11 and FIG. 12 show an embodiment of the joint (10) between thebarge (2) and the lifting construction (1) according to the invention.On the figures it is shown that the joint is placed in the skidding rail(11) attached to the deck (24). The joint comprises a plate (20) that issecured to the arm (1) that is pivotally secured to a skidding shoe (21)that again is skiddingly or glidingly attached in the skidding rail(11). The joint (10) is shown with a bolt/bearing assembly (23). Theskidding shoe (21) can be locked to the skidding rail (11) by means of alocking device (22).

A method for the use of the arm (1) according to the invention as shownon FIG. 1 to FIG. 4 includes to guide the barge (2), with the arm (1),to a jacket on an offshore installation (13) that is to be removed. Thearm (1) is then ballasted, preferably by filling ballast tanks (notshown) in the arm (1) with sea water, such that the arm (1) sinks andpivots about the joint (10). The ballasting can be performed by lettingvalves in the arm open, or that water is pumped into the ballast tankswith suitable pumps. The ballasting is preferably performed in asequence in relation to the various ballast tanks such that the loweringcan be performed with a controlled load on the joint (10) or the barge(2).

When the arm (1) is lowered to a vertical position (FIG. 2), the jacket(9) is placed between the two branches (8) and is secured with a holdingarrangement (9) to the arm (1). The jacket (13) is then released fromits foundations (not shown). The intermediate supporting elements (6)are adjusted, or are adjusted in advance, to bear against strong points(14) on the jacket (13).

Subsequently the ballast tanks are deballasted such that the arm (1)pivots along with the jacket (13) about the joint (10). The ballastingis preferably performed sequentially such that the point of applicationof the buoyancy is placed such that the arm pivots about the joint (10)without any unwanted forces being applied. The pivoting movement iscontinued until the arm (1) is substantially placed horizontally as anelongation of the barge (2), as shown on the FIG. 3.

The barge (2) is then pulled in below the arm (1) by means of winches(12) and wires (16) as shown on FIG. 4. The arm (1) is then ballasted orsecured to the barge (2) such that these are stably placed in relationto each other. The barge (2) with the arm (1) and the jacket (13) canthen be moved to a suitable place.

A method for using the embodiment shown on FIG. 5 to FIG. 10 has manycommon features with the method as described above, but the arm (1) isplaced on a vessel (3) with a u-shaped end and is instead pivoting abouta joint (18) placed between the arm (1) and the u-shaped end. Duringballasting and deballasting, as described for the embodiments on FIG. 1to FIG. 4, the arm will pivot about a joint (18). If the vessel issemi-submergible, it can be adjusted in terms of its position in thewater to adjust the distance between the supporting elements (6) and thestrong points (14) on the jacket (13), and to participate is duringraising or lowering of the jacket (13) and the arm (1).

Alternatively this method can be reversed for the launching ofinstallations.

The present application is not intended to be limited to the embodimentsdescribed above, and shall only be considered limited by the enclosedclaims.

1. An offshore lifting construction for the lifting of jackets or jacketlegs for offshore platforms, the lifting construction comprising: aframe with means for ballasting for raising and lowering the frame, theframe being adapted to take a substantially horizontal raised positionand a substantially vertical submerged position in relation to a watersurface where a jacket is placed, and including a lower end and an upperend and at least two, in a longitudinal direction of the frame,substantially parallel first longitudinal elements with intermediatesupporting elements for the jacket, and the frame being equipped withmeans for attachment of the jacket that is to be lifted, wherein theframe has a lower tubular transversal unit located at a bottom of thelower end of the frame; the frame includes means for pivotal attachmentto a vessel; the lower end of the frame includes at least twosubstantially parallel second longitudinal elements, each having onefree end and one end attached to the lower end of the frame, the atleast two substantially parallel second longitudinal elements beingsubstantially perpendicular with the substantially parallel firstlongitudinal elements and a longitudinal direction of the lower tubulartransversal unit, such that the at least two second longitudinalelements and the lower tubular transversal unit are adapted for placingthe jacket therebetween during lifting; the substantially parallel firstlongitudinal elements, the lower tubular transversal unit and the twosubstantially parallel second longitudinal elements include separateinternal ballast tanks; and the frame includes a second frame having themeans for attachment of the jacket that is to be lifted arranged at theupper end of the frame, and fixed between the substantially parallelfirst longitudinal elements.
 2. An offshore lifting constructionaccording to claim 1, where the means for attachment of the jacket thatis to be lifted includes winches for raising the jacket in relation tothe frame.
 3. An offshore lifting construction according to claim 1,wherein the means for attachment of the jacket that is to be liftedincludes a passive wire system and deballasting alone is used for thelifting.
 4. An offshore lifting construction according to claim 1,wherein the frame is adapted to be led over the vessel.
 5. An offshorelifting construction according to claim 4, wherein the vessel compriseswinches with wires adapted to pull the frame over the vessel.
 6. Anoffshore lifting construction according to claim 4, wherein the vesselincludes a skidding rail to lead the frame over the vessel.
 7. Anoffshore lifting construction according to claim 4, wherein the meansfor pivotable attachment to the vessel comprises a plate secured to theframe, a skidding shoe pivotably attached to the plate, and a lockingdevice for securing the skidding shoe to the skidding rail where theskidding shoe is glidingly connected to the skidding rail.
 8. Anoffshore lifting construction according to claim 1, wherein at least oneof the intermediate supporting elements can be adjusted in relation tothe longitudinal direction of the frame such that the at least one ofthe intermediate supporting elements can be directed towards strongpoints on the jacket that is to be lifted.
 9. A method for raisingjackets or jacket legs for offshore platforms by means of an offshorelifting construction according to claim 1, the method comprising:leading the vessel with the frame to the jacket of an offshoreinstallation that is to be removed; ballasting the frame by filling theballast tanks in the frame with sea water, in a sequence in relation tothe various ballast tanks such that submersion is performed with a loadon the means for pivotal attachment and the vessel within predeterminedlimits, such that the frame sinks and pivots about the means for pivotalattachment to a submerged substantially vertical position; placing thejacket between the at least two substantially parallel secondlongitudinal elements; securing the jacket with the means for attachmentof the jacket to the frame; and deballasting the ballast tanks such thatthe frame is pivoted along with the jacket about the means for pivotalattachement until the frame lies substantially horizontal and the jacketis raised.
 10. A method according to claim 9, wherein the frame isadapted to be led over the vessel, the vessel comprises winches withwires adapted to pull the frame over the vessel and a skidding rail forguiding the frame over the vessel, and the method further comprises:pulling the vessel in below the frame by means of the winches and thewires.